The present invention relates to an apparatus for aerating and circulating a liquid and, in particular, water in a pond.
Various devices exist for the introduction of air bubbles into a mass of liquid, typically water. Such devices have numerous applications which encompass areas such as aquaculture, including culturing and raising fish in controlled environments (known generally as fish farming), waste water management and the like. Other uses are generally known in the art. Such devices include what are generally known as surface aeration devices and air lift devices.
Surface aeration devices in the art use a form of paddle wheel to agitate the water near the surface thereof. This causes bubbles of ambient air to become trapped in the water, some of which are absorbed into the water. The paddles further force the aerated water away from the device and draw additional water toward the device to be aerated. Surface aerators are inefficient, requiring a large energy input relative to the volume of water aerated. They also have several moving parts which can require frequent maintenance.
Air lift devices have been known for many years and essentially operate by supplying air bubbles into water at a predetermined depth below the surface. Some of this air is absorbed into the water, which causes the water to become less dense and rise toward the surface. The rising of the water causes circulation thereof, which distributes the aerated water and brings additional water toward the device for aeration. Water is aerated in an air lift device by the use of a diffuser. Many forms of diffusers have been in existence for years and typically include a porous body through which air or another gas such as nitrogen or oxygen is forced. When the diffuser is submerged in water, the movement of gas through the device causes bubbles to emerge from the pores and into the water.
Known diffusers include ceramic dome diffusers and porous rubber hoses. However, it has been found that dome diffusers are difficult to maintain, lack durability, and are more costly to manufacture in mass production.
Porous rubber hoses have been used for aeration. However, a key objection to off-the-shelf porous rubber hoses and other porous rubber aeration products is that, after a cure period in water, the hose requires high air pressure delivery systems because the rubber particles unsatisfactorily swell in water thereby decreasing pore diameter. Decreasing pore diameter causes increased flow resistance and uneven aeration patterns in long runs of tubing become more problematic as the pipes age in their environment. Further problems associated with the manufacture of porous aeration pipe utilizing previous methods have included irregularly shaped pipe walls, inconsistent porosity, and ineffective micropore size and wall thickness producing inconsistent and unreliable aeration rates.
Porous hoses are available which are made from thermoformable polymeric material, such as polyvinyl chloride (PVC) and polyethylene (PE). These types of porous hoses are formed by dispersing air bubbles into the thermoformable material while in a molten state, thereby forming a foam-like material which is then extruded into the shape of a hose, leaving the air bubbles therein when the structure freezes. In such a structure, some of the air bubbles will create pores which extend from the inside of the hose structure to the outside thereof, providing a path for gas to diffuse into water outside the hose. However, the air bubbles are irregularly distributed throughout the body of the hose, leading to irregular size and placement of the pores. For example, some air bubbles may be completely contained within the wall of the hose or may only be exposed to one side of the wall, neither of which creates a pore. Additionally, it may take multiple bubbles being linked together to actually form a passage completely through the hose, which increases the resistance of the path. Both of these conditions can lead to inefficiencies, which can reduce the efficacy of an air lift which employs such a diffuser.
Generally, the size of the air bubbles which are diffused into water during aeration thereof can be varied by providing a diffusion device with smaller or larger pores formed therein, smaller pores producing smaller bubbles. Some known devices favor the use of smaller bubbles, because smaller bubbles are absorbed more readily into water. However, for reasons described above, many known devices for diffusing fine air bubbles into water are expensive or inefficient to operate. Other known devices favor the use of larger, more coarse bubbles because such bubbles travel through water at a higher velocity.
Therefore, there exists a need for a device which is useful for aerating a large volume of liquid that does so efficiently while utilizing a less-expensive or more reliable diffusion device that provides for adequate absorption of gas into the liquid medium.